This invention relates generally to the measurement of physical mechanical properties of fluid samples and, more specifically, to the time varying viscoelastic characteristics of a coagulating blood sample.
Measurement of the physical properties of fluids is required in many fields, such as the medical, food processing, chemical processing, and various manufacturing fields, for example. One prior art instrument for performing such fluid measurements is the Sonoclot Analyzer, manufactured by Sienco, Inc. of Morrison, Colorado. The Sonoclot Analyzer incorporates a mechanical oscillatory probe designed to oscillate at a resonant frequency of a sensor, circuit and fluid sample. This instrument generates an output signal responsive to both viscous and elastic characteristics of a fluid sample being analyzed. This output signal is plotted against time to generate a graph or signature of the changing viscoelastic characteristics of the fluid sample under test. The Sonoclot Analyzer exhibits several shortcomings. The measurement of blood coagulation characteristics, for example, is temperature sensitive. The temperature control in the prior art instrument suffers from a nominal droop error of 0.1 degree C. per degree C. change in ambient temperature. For surgical applications, typical ambient temperatures lie in the range of 12 degrees C. to 30 degrees C. The resulting sample fluid temperature error of about plus or minus 1 degree C. can corrupt the measurement and diminish confidence in the test results. In addition, manual interpretation of the graphical output signal of the prior art analyzer is both difficult for many surgical personnel and is prone to human error. Also, the analog output signal of the prior art instrument can be corrupted by mechanical noise. Simply bumping this instrument will create a noise spike on the output signal. Even noise produced by footsteps or normal conversation appear on the output signal and further complicate the required manual interpretation of the graphical output signal. Yet another shortcoming of the prior art instrument is its complicated signal conditioning circuitry and the complexity of its calibration procedure. This instrument requires high interchangeability between transducers to ensure the desired oscillation. Often performance parameters cannot be met without altering control circuit component values. Also, the oscillator within this prior art instrument incorporates a transconductance amplifier configured as a variable gain amplifier. The transconductance gain exhibits excessive temperature drift and is not referenced to a known zero gain voltage. Consequently, the transconductance gain cannot be used as an output signal. Instead, an output signal must be constructed from an AC signal within the oscillator feedback loop. This additional circuitry increases complexity and cost of the instrument.
It is therefore a principal object of the present invention to provide an instrument for testing the viscoelastic characteristics of fluids that (1) exhibits improved temperature regulation., (2) includes improved probe drive circuitry to simplify calibration and signal conditioning; (3) incorporates resonant frequency monitoring to detect mechanical noise disturbances; and (4) provides automated analysis of the graphical output signal to simplify use of the instrument by surgical personnel and to improve its accuracy.